Hrimthur possessed excellent masonry skills courtesy of his father, Thamur , who was renowned as the greatest stonemason in the world. To his father's dismay, Hrimthur had the heart of a warrior and refused to assist his father in building the walls around Jötunheim. A quarrel between father and son spiraled out of control and Thamur struck his son, causing Hrimthur to run away.
Thamur was killed by Thor while attempting to locate his son.
Hrimthur ultimately completed his father's work, the Great Walls of Jötunheim. He then desired vengeance against the Aesir for their crimes against the Giants. He initially wanted to fight Thor but the tragedy that befell his father taught him otherwise. Hrimthur observed that Asgard's walls were "half-built and shoddy" and so he adopted the guise of an ordinary mortal and made the Aesir an offer: he would build them majestic new walls, and if he couldn't build them within two years, they would owe him nothing. But if he succeeded, he asked only for an audience with the goddess Freya.
Odin agreed, believing the task to be impossible and out of curiosity towards the stranger. With the lessons taught by his father and the help of a magical stallion that he used fetching stones, Hrimthur completed the task, much to Odin's frustration. Odin seemingly agreed to uphold his end of the bargain and sent Freya to speak with Hrimthur. Hrimthur whispered something into the goddess's ear and headed for the entrance to Asgard, where Thor waited for him.
It was then that Hrimthur realized he had been double-crossed and would be slain by Thor like his father and most of his kind had. But he did not care, for his plan was complete. Mimir suspects that Hrimthur had embedded some weakness into the Aesir's walls and passed this knowledge onto Freya, who had little affection for the Aesir, in preparation for their downfall at Ragnarök.